1. Field of the Invention
The present invention relates to a deflection system and more particularly to digital-to-analog (D/A) converters in the deflection system of an electron beam exposure device.
2. Description of the Prior Art
Digital-to-analog converters used in the deflection system of an electron beam exposure device must be capable of converting data consisting of 18 to 20 bits. However, conventional D/A converters are simply contructed only to a degree which renders them capable of converting data consisting of 12 to 14 bits. In conventional D/A converters, therefore, data consisting of the most significant bits, i.e., m bits, is decoded so as to turn on S constant-current (voltage) sources among 2.sup.m constant-current (voltage) sources, and data consisting of the least significant bits, i.e., n-m bits, is input into the D/A converters so as to add the outputs in an analog manner.
In the above-mentioned system, however, the mismatching of the weighting of bits occurs when the data changes, for example, from 01111 to 10000, and deflection of the electron beam must be interrupted when the mismatching phenomenon occurs. In the above-mentioned system, furthermore, the mismatching of bits occurs due to the mismatching of the weighting of the resistances of switching transistors which work to convert binary signals into analog signal, and the pattern which is described becomes locally narrowed or widened. Further, a Gritch phenomenon occurs due to the simultaneous switching of data having different weighting values.
Namely, in the above-mentioned conventional system, it is difficult to maintain a monotone increase, which is the most important requirement for an electron beam exposure device. To maintain a monotone increase, it is necessary to finely adjust the output of the constant-current (voltage) source.